The present invention relates to an electrophotographic photoreceptor including oxytitanium phthalocyanine which is in specified crystal form, as a charge-generating agent, and including a specified compound as a charge-transfer agent.
As the exposure source for non-impact printer adopting electrophotographic method, long-wavelength light sources (such as semiconductor laser and LED) have mainly been applied in recent years. With the current movement of miniaturization and increased speeds for copiers and printers, a reduction in diameter of photoreceptor and a process having high circumferential speed have been adopted. As a result, a charge-generating agent having sensitivity in long-wavelength region is generally used for the electrophotographic photoreceptor. Conventionally, this type of material often adopts a phthalocyanine-based pigment. The phthalocyanine-based pigment is known to have different sensitivities depending on the crystal form thereof. In addition, along with the power-saving trend in recent years, the electrophotographic photoreceptor has been facing increasing requirement for higher sensitivity than ever to suppress the output of exposure source of electrophotographic apparatus such as a printer.
(1) As of phthalocyanine-based pigments, the one which has high sensitivity in a long-wavelength region includes oxytitanium phthalocyanine. Although there have been many crystal forms introduced for this oxytitanium phthalocyanine, the one having a maximum diffraction peak at 27.2° is accepted as highly-sensitive. If, however, oxytitanium phthalocyanine is used in a high-speed process, the potential characteristics of the photoreceptor become deteriorated after repeated use, and fog, black stripes, uneven concentration, or the like occurs in the formed image.
These phenomena presumably come from relatively large quantity of charge generated due to the high sensitivity characteristic of the oxytitanium phthalocyanine. That is, although the large quantity of generated charge normally has advantages such as high response, the charge remains in the photosensitive layer and remains in memory on the photoreceptor when the oxytitanium phthalocyanine is used in a high-speed process, thereby creating images as a memory phenomenon in the succeeding electrophotographic process. There is also an effect due to the charge transport capacity of a charge-transfer agent that is present; thus, the combination of both of them is important, for example, refer to JPA No. 1-106069
Consequently, it has been necessary to have an electrophotographic photoreceptor, which has high sensitivity in the long-wavelength region and which maintains the stability of electrophotographic characteristics even after repeated use at high speed, specifically the stability of reproducibility of an initial potential for a potential after repeated use. Even when a charge-generating agent having high charge-generating efficiency is used, if the compatibility with the charge-transfer agent is poor, satisfactory sensitivity cannot be attained and high quality image cannot be formed in various environments, when in use, ranging from high temperature and high humidity to low temperature and low humidity. Although compatibility between the charge-generating agent and the charge-transfer agent has been studied from various standpoints, it has not yet been definitely formed.
(2) On the other hand, various methods for manufacturing electrophotographic photoreceptor have been studied. A general method therefor is to disperse the charge-generating agent, the charge-transfer agent or the like in a solvent together with a binder resin to prepare the coating solution, and then apply the coating solution on a conductive support member to form a thin film.
In a general practice for forming the charge-transfer layer is to dissolve the charge-transfer agent and the binder resin in a coating-preparation solvent in order to form the coating solution, and then to apply the coating solution on a conductive support member, followed by drying the solution.
The charge-transfer agent, however, is difficult to be fully dissolved in varieties of solvents, and is also difficult to be fully dissolved in varieties of binder resins.
Conventionally, the use of methylene chloride or dichloroethane is examined as a coating-preparation solvent. These coating-preparation solvents are considered to have relatively high solubility to the above-discussed charge-transfer substances or binder resins, and to have a low boiling point in order to easily attain uniformity of coating film thickness and to make it easy to conduct drying (see, for example, JPA No. 2001-125288, JPA No. 2000-314977 and JPA No. 2004-354673).
However, when methylene chloride or dichloroethane is used as a coating-preparation solvent, there arises a problem of generation of portions decreasing locally the charged voltage in the organic photoreceptor, and of deterioration of the image quality by image noise, unless heating and drying are fully given after the application and formation of the charge-transfer layer to completely evaporate the coating-preparation solvent.
Moreover, in order to solve this problem, for a long period of heating and drying, there arises another problem of crack generation on the charge-transfer layer, thereby generating image noise, and a problem due to the difficulty in determining adequate drying condition, thereby making it difficult to increase the mass production yield.
(3) Recently additional electrophotographic apparatuses (such as, digital copier and printer) have been widely used, and the requirements for high image quality, miniaturization, and high speed have been further increasing.
Particularly for the high speed copier, which has short transition time from the exposure step to the development step, a problem occurs due to the failure in providing clear reproduction of dot image and fine lines. In response to this problem, there is proposed and commercially used a laminated electrophotographic photoreceptor which includes the photosensitive layer by allocating the functions, respectively, to the charge-generating layer that includes a charge-generating agent having high sensitivity in long-wavelength region and to the charge-transfer layer such that the charge-transfer agent having high plate life and high transfer degree is dispersed into the binder resin.